Engine-Out Emissions from a Direct-Injection Spark-Ignition (DISI) Engine 1999-01-1529
The effects of operating parameters (speed, load, spark-timing, EGR, and end of fuel injection timing [EOI]) on engine-out, regulated (total HC, NOx, and CO) and speciated HC emissions have been investigated for a 1.83 L direct-injection, spark-ignition (DISI) engine. As the EOI is varied over the range from high to low stratification with other engine parameters held constant, the mole fractions of all regulated emissions vary sharply over relatively small (10-20 crank angle degrees [CAD]) changes in EOI, suggesting that emissions are very sensitive to the evaporation, mixing, and motion of the stratified fuel cloud prior to ignition. The contribution of unburned fuel to the HC emissions decreases while the olefinic partial oxidation products increase as the fuel stratification increases, increasing the smog reactivity of the HC in the exhaust gas by 25%. Varying the engine speed at fixed EOI demonstrates that the emissions trends are strongly affected by the degree of stratification. For early fuel injection (low stratification), increasing the speed causes the HC emissions to decrease and the NOx emissions to increase similar to homogeneous-charge SI engines. For late fuel injection (maximum stratification), opposite trends are observed - increasing speed results in lower NOx and higher HC emissions. These results show that the details of the fuel cloud evaporation, mixing, and location relative to the spark plug strongly influence the emissions from DISI engines. Furthermore, conventional (homogeneous-charge) wisdom regarding engine emissions is not necessarily applicable to DISI engines operated with high fuel stratification.